A conventional method for such layer transfer is known as the “Smart Cut™” method; it consists mainly in carrying out the following steps (see in particular the French Patent Application No. FR-2 681 472 or its equivalent U.S. Pat. No. 5,374,564 and its various developments and improvements):
creation by ionic implantation of a buried weakened zone within the source substrate, delimiting with the free surface the future thin layer to be transferred,
assembly of the source substrate and the target substrate at said free surface, and
input of thermal and/or mechanical energy to provoke a fracture in the weakened zone within the source substrate.
During fabrication by this “Smart Cut™” method of a heterostructure (in particular, a structure made up of at least two different materials, generally in a plurality of layers, and having a thickness typically between 1 μm and 1 cm inclusive), control of internal stresses is very important if the materials of the heterostructure have significantly different coefficients of thermal expansion and it is required to induce the fracture at a temperature significantly different from that at which bonding was effected (for example when it is required to use a heat treatment to induce some or all of the fracture in the weakened zone).
For example, in the case of the transfer of a film of silicon from a source substrate (of which at least a surface portion is in silicon) onto a target substrate the coefficient of thermal expansion of which is very different from that of the silicon source substrate (for example a fused silica target substrate), the two solid substrates are conventionally bonded at room temperature, for example by molecular bonding. When, to transfer the film, the choice is made to use an input of thermal energy, it is known that the bonding interface is then consolidated; however, this heat treatment also has the effect that internal stresses, which can be very high, are generated as a consequence of the difference between the coefficients of expansion on either side of this bonding interface; it follows from this that when the transfer of the silicon film is effected (in particular, when the fracture induced by the “Smart Cut™” method occurs), the two substrates (or a portion of the two substrates if the fracture does not extend over the whole area of the substrates) are brutally separated and then immediately relax. This stress jump, if it is of too high a magnitude, risks damaging one or the other of the two parts of the heterostructure separated in this way (formed, in the example considered here, by the silica substrate carrying the transferred thin film of silicon and the silicon substrate in which the fracture has been provoked).
There would be a benefit in being able to minimize the stress jump that occurs on the separation of a heterostructure at a temperature different from its creation temperature.
To minimize any such stress jump, thought may be given to creating the heterostructure at a higher temperature, preferably at least approximately at the temperature at which the fracture is subsequently to be provoked. However, when the heterostructure is produced by molecular bonding, the bonding energy decreases greatly when bonding above 200° C., although this is a low temperature at which application of the “Smart Cut™” technology can prove difficult, simply by input of thermal energy, in a silicon/fused silica system, for example (to transfer a silicon film onto a fused silica substrate); it follows from this that, when it is required to provoke fracture only by input of thermal energy, it is required in practice to proceed at a temperature much higher than 200° C. Now, if the bonding energy is too low, the stresses of thermal origin can be sufficient to provoke unsticking of the structure at the interface (rather than in the weakened zone) or at least lead to poor functioning of the “Smart Cut™” technology: the bonding interface may then not withstand the vertical pressure imposed by the development of microcavities that this method generates (on this subject see “Silicon on insulator material technology”, M. Bruel, Electron. Lett. Vol. 31-No. 14 (1995) p. 1201).
To minimize the stress jump it has already been proposed to bond the parts of the heterostructure under conditions such that the stress regime at the interface falls below a given threshold when this heterostructure is brought to the temperature at which it is wished to provoke the fracture in a weakened zone in one of the wafers near the bonding interface. Thus French Patent Application No. FR-2 848 336 or its equivalent U.S. Patent Publication No. 2006/0205179 proposes to effect the bonding of two wafers that have been subject beforehand to deformation. To be more precise, the above document teaches imposing a stress on the wafers at the moment of bonding at room temperature by bending the two plates before molecular bonding; if the curvature is carefully chosen, it is possible to minimize or even to eliminate internal stresses generated by thermal annealing of the heterostructure at the fracture temperature. However, to enable separation by the “Smart Cut™” method during thermal annealing of short duration, this method generally calls for bonding the structures with fairly high radii of curvature which, from the technological point of view, can prove relatively difficult to achieve on an industrial scale; moreover, the conditions of the future fracture must be known at the time that bonding is effected. On the other hand, this technology has the advantage that the molecular bonding can be effected at room temperature and thus makes it possible to have a good bonding energy at the moment of the transfer.
It follows from this that it is therefore possible to transfer a thin layer at a temperature as high as may be required from one of the wafers to the other wafer, the fracture occurring in the weakened zone previously formed, whereas the bonding interface between this thin layer and the wafer to which it is henceforth fixed can have a high bonding energy. It must nevertheless be noted that, on returning to room temperature, the thin layer may be stressed in traction or in compression because its coefficient of thermal expansion is different from that of the wafer to which it has been firmly attached by molecular bonding; because the target wafer is in practice more solid than the thin layer, it is hardly deformed when the temperature changes after fracture, imposing a change in the dimensions of the thin layer because of the change of temperature.
This kind of phenomenon had already been exploited in the case of a homostructure (in particular, a structure formed of layers or substrates in the same material); Feijoo et al. have proposed to impose a stress within a homostructure at room temperature by application of a deformation just before the formation of this homostructure by bonding (see D. Feijoo, I. Ong, K. Mitani, W. S. Yang, S. Yu and U. M. Gösele, “Prestressing of bonded wafers”, Proceedings of the 1st international symposium on semiconductor wafer bonding, Science, Technology and Applications, Vol. 92-7, The Electrochemical Society (1992) p. 230). To be more precise, in the above paper, a homostructure consists of two silicon wafers that are bonded with a certain radius of curvature (the paper states that the plates are bonded and unbonded several times during deformation). The authors propose thinning one of the silicon crystals thereafter at room temperature by mechanical means (lapping) so as to be able to impose a high stress in the thinned silicon film after return of the other plate to a plane shape.
It should be noted that the above document does not envisage obtaining the thin layer by fracture within one of the plates of the homostructure; a fortiori, the above document does not address in any way the problem of the separation of a heterostructure at a temperature different from the creation temperature (in fact, there would have been no particular problem with regard to a homostructure, since there is no thermal effect on the stress state at the interface).